(1) Field of the Invention
The invention relates to hydrophones and is directed more particularly to a differential hydrophone assembly for transforming acoustic waves into electrical signals without undue sensitivity to common mode noise.
(2) Description of the Prior Art
Hydrophone assemblies historically have included a piezoelectric hydrophone electrically connected to a hydrophone amplifier. As ships and SONAR systems became larger, the amplifiers were placed remotely from the hydrophones and connected to the hydrophones by way of long cables which suffered signal loss. To reduce the signal loss a first stage (preamplifier) of the amplifier was moved closer to the hydrophone. More recently, a differential input preamplifier has been developed to reduce the effect of common mode noise. In common SONAR environments electric and electronic devices produce electric and magnetic (EM) fields that produce common mode noise on a ground reference of a SONAR system, while the water environment of the hydrophone is relatively quiet and unaffected by EM field generated noise.
Although the historical hydrophone assembly configurations have not changed substantially in over fifty years, the SONAR electronics and other shipboard electronics have become quite complex and have become a major source of electromagnetic interference (EMI). The EMI has corrupted the ground reference to the amplifier, producing a large common mode noise on the reference ground plane. This common mode noise is reflected to the input of the amplifier, such that even a differential preamplifier is affected by the presence of the common mode noise.
Further, prior art hydrophones include a substantially annular ceramic member with a positive electrode on an external surface thereof and a negative electrode on an internal surface thereof. This configuration produces an unbalanced common mode current and reduces the amplifier capability to reject common mode noise. The amplifier cannot fully null the common mode noise.
Accordingly, there is a need for a differential hydrophone assembly which nulls common mode noise and generates an electric signal with a high degree of immunity to EM fields.